1. Field of the Invention
The present invention relates to a substrate processing method for processing a substrate. Substrates to be processed include, for example, semiconductor wafers, liquid crystal display device substrates, plasma display substrates, FED (Field Emission Display) substrates, optical disk substrates, magnetic disk substrates, magneto-optical disk substrates, photomask substrates, ceramic substrates, and photovoltaic cell substrates.
2. Description of Related Art
In a process of manufacturing semiconductor devices and liquid crystal display devices, etching treatment is performed as necessary by supplying a high-temperature phosphoric acid aqueous solution as an etchant onto the front surface of a substrate, having a silicon nitride film (SiN) and a silicon oxide film (SiO2) formed thereon, to selectively etch the silicon nitride film. Japanese Patent Application Publication No. 2012-074601 discloses that a phosphoric acid aqueous solution close to the boiling point is supplied to a substrate held by a spin chuck and the phosphoric acid aqueous solution on the substrate is thereafter rinsed off by pure water.
The phosphoric acid aqueous solution that is supplied to the substrate ordinarily contains silicon. If the concentration of silicon is within an appropriate range, the etching selectivity ((etching amount of the silicon nitride film)/(etching amount of the silicon oxide film)) is increased. However, when pure water at room temperature is supplied to the substrate after the high-temperature phosphoric acid aqueous solution has been supplied, the temperature of the phosphoric acid aqueous solution on the substrate decreases and the saturation concentration of silicon decreases. The concentration of silicon in the phosphoric acid aqueous solution thus becomes not less than the saturation concentration and silicon precipitates from the phosphoric acid aqueous solution. Precipitated matter that includes the silicon remains on the upper surface of the substrate, thereby contaminating the substrate.